| With extremely wide operating band, ultra-wideband (UWB) technology can be used in high-speed data rate transmission of no carrier. Its ultra-low emission power and insensitivity to multipath effects make it more convenient for indoor wireless communications. Multi-input Multi-output (MIMO) technology can increase channel capacity without sacrificing additional spectrum resources and total transmission power, and it also can improve channel reliability, resist to multipath fading and reduce error bit rate. For a long time, UWB and MIMO technology has been as two individual solutions for future wireless communications. With the development of high-definition television (HDTV) and high-speed short-range communication, it is believed that the indoor data rate will reach as high as 1Gb/s in the near future. To meet the future needs of high-speed communication, a new approach that combining UWB and MIMO technology is proposed. By taking full use of space diversity of MIMO system and wide frequency band of UWB technology, UWB-MIMO system can reach speed of Gb/s level. In this paper, author has studied compact UWB antenna, its notch band technology, multi-element technology and its time-domain characteristics analysis. The main aspects of this paper can be concluded as follows:1. Several forms of compact UWB antnenas are designed, including half-moon-shape UWB antenna with narrow ground plane, fan-shape UWB antenna with CPW-fed, quasi-self-complementary antenna (QSCA) with UWB performance. Then, their impedance bandwidth, gains, radiation patterns, antenna efficiencies and group delays are measured and analyzed. By comparing simulated and measured results, the proposed UWB antennas show good performaces which can meet the demands of future UWB wirelss communications.2. Based on the semi-circular patch antenna with three kinds ground planes and quasi-dipole with two kinds of ground planes, the impact of ground plane and radiation patch on the UWB antenna time-domain behaviors are studied. By placing virtual probes at six fundamental directions of the antenna, the transmission properties are analyzed, including signal waveforms, power spectrum and fidelity factors. By comparing the probe-received and the original signals, although with different radiation patches, the UWB antenna with triangular cut ground plane has better time-domain behaviors than others.3. Several different forms of notch band technologies have been proposed, including meandering slots loading, dual L-shape grounded branch, inverted T-shape stub and quarter wavelength paralell microstrip lines with open-ended. By adjusting the length of the slot and the distance between the slots and the patch edge, the desired notch band can be obtained. By adjusting its own length and the distance between branch and ground plane, the dual L-shape grounded branch can generate two desired notch bands; while its position on the circle affects a little on its notch band characteristics. The T-shape stub notch band come from its open-ended resonating, but its physical size make it need a large enough patch where the stub can be fully inserted; The open-ended quarter-wavelength line creates a short path between the ground plane and the radiation patch, and then by tunning the length of the other open-ended line, a desired notch band can be achieved.4. Based on the electric small antenna, two novel UWB-MIMO antenna are proposed. Firstly, a UWB-MIMO antenna with meandered monopole is proposed; its ultra-wide band is obtained from the capacitive coupling between the end-shorted L-shape strips and meandering monopole; then, by introducing a wide gap between the two antenna elements, a higher isolation can be achieved; and considering its practical application scenarios, its work status near the head is analyzed. Secondly, a UWB-MIMO antenna with dual notch band is proposed, the loaded T-shape grounded branch enables its coupling reduction; To meet the future demand of the UWB communications, the UWB-MIMO antenna with dual notch band characteristics is also implemented.5. According to the radiation characteristics of the QSCA, a two-element UWB-MIMO antenna is presented and its diversity performance, group delay and working status near the human head are all analyzed; by comparison, it can be seen in the absence of any decoupling methods and structures, the UWB-MIMO antenna based on QSCA can reach a high isolation, which both can cater the UWB coverage and diversity performance demand in the future wireless communications; then, on this basis, two more kinds of four-element UWB-MIMO antenna with elements parallel and anticlockwise arranged, are proposed, simulated and disscused.
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